## Story Time
Metadata API introduced two exports `metadata` and `generateMetadata` to the pages and layouts under app router, as partial prerendering work is going on and people are desiring to render the metadata asynchronizly, this change will be the preparation for moving to the dynamic & asynchronized land. In short, if we can render the metadata asynchronizedly, it will benefit the performance of the initial page loading and client page transiation a lot. Any slow data fetching can be handled while the essential page "shell" is rendered.
For meta tags, there're few ones will visually affect your web page, such as `<meta name="viewport">`, `<meta name="theme color">` and `<meta name="color-scheme">`, rendering them lately after the page frame is ready might bring flickering to the page such as revreting whole page's theme color or shaking due to viewport updates. Those meta are not majorly the "metadata" for SEO, but more for user experience when opening the page. If we're rendering everything as async meta tags, it won't be ideal due to the flickering on your web pages.
## Solution Preparation
We'll want to render the meta tags separately to make sure the visual ones are rendered as blocking along with web page, and then the ones for SEO or bots can be flushed later by later, like a suspense boundary keeps emitting them into the head of html.
We optionally picked up 3 meta tag "viewport", "theme-color" and "color-scheme" to be render first into the web page with html "shell", to guarantee the layout viewport and basic styling are rendered first.
This PR introduced two module export in the page and layout files: `viewport` and `generateViewport`, in order to separate the visual meta tags from the SEO metadata.
### API
```ts
// page.js | layout.js
export const viewport = {
// viewport meta tag
width: 'device-width',
initialScale: 1,
maximumScale: 1,
interactiveWidget: 'resizes-visual',
// visual meta tags
colorScheme: 'dark',
themeColor: { color: 'cyan', media: '(prefers-color-scheme: dark)' },
}
```
There's also a dynamic API like what we did for metadata API
```ts
// page.js | layout.js
export function generateViewport() {
return { ... }
}
```
## Notice
This PR won't get SEO metadata rendered asyncronizedly, instead it's a preparation for the later work in partial prerendering and async metadata. We'll encourage the Next.js community moving to the new metadata viewport API if you're customzing those 3 meta tags. Usually you don't have to change viewport itself, so mostly like only theme-color and color-scheme could relate to it.
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